Thin film semiconductor materials, and amorphous silicon in particular, have emerged over the past several years as prime candidates for use in low cost, large area thin film electronic applications. They have attractive electronic characteristics, are relatively simply fabricated by the plasma chemical vapor deposition (CVD) method, and are capable of large area (in excess of 12 inches by 12 inches) deposition upon inexpensive substrates, such as glass, at low processing temperatures. Initially amorphous silicon was used in photovoltaic applications and later in large area integrated circuits used in flat panel liquid crystal displays, solid state imagers, electronic copiers, printers and scanners. The relatively simple fabrication techniques enable the formation of active and passive electronic devices together with bus lines and interconnects on a single, page size (and larger) substrate. Such an array of thin film transistors may be used for driving active matrix liquid crystal displays as taught in a copending application U.S. Ser. No. 07/619,360, filed on Nov. 28, 1990 entitled "Timing Independent Pixel Scale Light Sensing Apparatus" (Hack et al.), assigned to the same assignee as the instant application. In a large area interactive display array, each pixel location would be provided with a control circuit including a phototransistor so as to be able to accept optical input for direct writing on the display with a light pen.
Amorphous silicon has been applied in solar cells and in photodiodes because of its high photoconductivity. In order to implement the optical input function, at the pixel level, in the above identified display, photosensors are incorporated into each control cell of the multi-cell array. Several factors enter into the selection of a satisfactory photosensor. It is important that the device has a large dynamic range (i.e. the ratio of photocurrent to dark current) so that it will be able to be switched ON and OFF reliably and rapidly. A large dynamic range will also allow many discernible levels of grey to be available, thereby enhancing the display output. Additionally, when a device exhibits high photoconductive gain it may be switched with lower levels of light which directly translates to lower cost lamps. Finally, low capacitance is often a desirable feature which is dictated by the circuit configuration in which it used.
It is well known that high photocurrent can be achieved by improving the charge injection efficiency into the photosensor by modifying the injecting contacts and by appropriately doping the semiconductor charge transport layer. However, both of these approaches cause the dark current to increase with the photocurrent, which could adversely affect the reliability of the OFF state of the device.
It is the primary object of the present invention to selectively utilize an offset gated, gap cell, thin film device as a photosensor in a high gain mode in which it will exhibit both low dark current and high photocurrent or in a unity gain mode in which it will exhibit an extremely high dynamic range and an extremely low dark current.